Such objects as the hulls of boats, cisterns, car-body components, and other similar objects are being made to an ever increasing extent from fibre-reinforced plastics materials, and particularly from polyester resin reinforced with glass-fibre. The normal method of manufacture is one in which the fibre-reinforcement and the plastics material are placed in layers on a mould, whereupon the plastics material is pressed into the fibre-reinforcement with the aid of a roller, or corresponding device for moistening the reinforcement throughout and for removing air inclusions. In an alternative method, plastics material containing chopped reinforcing fibres is injected, under high pressure, directly into the mould cavity, until a layer of the desired thickness is obtained.
Both of these methods represent serious health hazards especially when using polyester resins which contain harmful styrenes, these styrenes being released during the moulding process, to mix with the air in the workshop. Attempts have been made to overcome this problem, by installing powerful ventilating systems. Among other things, this has proved to be relatively expensive, since in addition to the costs involved in the actual ventilating apparatus, heating costs are also increased. In addition to costs, such systems create irritating noise and raise the level of draughts in the workshop.
In order to solve these problems, and in an effort to increase output, attempts have been made to manufacture products of the aforementioned kind in closed moulds from which gases can be readily removed, so as not to contaminate the air in the workshop. In the case of moulded shells, the moulds comprise a female mould-section and a male mould-section, the latter being brought into the female mould-section to a position in which the distance between the male and female sections of the moulds correspond to the desired thickness of the objects being produced. In one method of procedure, the plastics material is poured into the female mould-section and there subjected to pressure, in conjunction with the insertion of the male section of the mould. Alternatively, a sub-pressure can be created in the female mould-section and the plastics material drawn into the mould cavity as a result thereof, or the plastics material can be forced into the mould cavity by application of an external overpressure.
In order to obtain male and female mould-sections of satisfactory rigidity, surface fineness, and wear durability, the sections must be made of steel. Steel moulds, however, are excessively expensive when intended for manufacture in small series or for the manufacture of large objects, such as the hulls of canoes or dinghies.